Silicone based materials run the gamut from liquids such as silicone oils to hard Shore A durometer rated silicone rubbers, e.g. silicone gaskets. Towards the softer end of this spectrum are materials known as silicone gels which are below the measurment range of the Shore A durometer, but may be characterized by cone penetration (ASTM D217) or by Voland hardness. These systems can be formed in a number of ways. One method synthesizes the crosslinked polymer in the presence of an extender fluid, e.g. a non-reactive silicone oil, or an excess of a vinyl rich liquid silicone compound to create a soft fluid-extended system. Silicone gels generally have Voland hardness of about 1 to 25 grams and elongations greater than 300% and often substantially greater than 500%, e.g. greater than 1000%. Alternatively, silicone gel systems can be fabricated from mixtures of vinyl-containing polysiloxanes and hydride-containing polysiloxanes such that the final product has the appropriate hardness, elongation and tensile properties characteristic of a silicone gel. Suitable examples of both types of gels are taught in, inter alia, U.S. Pat. Nos. 4,600,261; 4,634,207; 5,140,746; 5,357,057; 5,079,300; 4,777,063; 5,257,058; 3,020,260; and Sylgard 527. Each of the preceding patents are completely incorporated herein by reference for all purposes.
U.S. Pat. No. 4,824,616 teaches making elastomer-skinned silicone gel particles by a two-step process involving (a) delivery of a silicone based material through a nozzle kept at very low temperature into a heated water bath optionally including a surfactant, and (b) forming an elastomeric layer on the surface of the silicone. gel particle. In particular, U.S. Pat. No. 4,824,616 mentions but does not exemplify that it is possible to crosslink the surface of a silicone gel particle bypassing it through a solution of organohydrogenpolysiloxane in a heat transfer medium, such as a silicone fluid. U.S. Pat. No. 5,124,090 teaches making silicone gel spheres by the delivery of a silicone based material through a nozzle, kept at very low temperature, into a heated water bath optionally including a surfactant.
The need to maintain a low viscosity of the mixed precursor materials in U.S. Pat. No. 4,824,616 and U.S. Pat. No. 5,124,090 requires mixing. and storage temperatures of -60.degree. C. to +5.degree. C. and preferably -10.degree. C. However, no teaching is given which indicates that angular complex shapes, as opposed to simple filament-like shapes, can be extruded. Neither of the above mentioned patents teaches the benefits of utilizing a higher temperature heat transfer medium, e.g. silicone oil, which enhances the processability and the rate of article formation. Most notably absent in the patent is the appreciation and the benefits of handling tacky gels through the use of a heated silicone oil bath, as well as examples on how to effectively use such a medium. Additionally, there is no recognition of the need for higher viscosities to create complex extruded shapes or how to achieve higher viscosities through the use of accelerators such as a catalyst and/or an increase in the reaction temperature, or the addition of fillers, or combinations thereof.
U.S. Pat. No. 4,783,289 teaches a process for the molding of silicone rubber compositions. Whereas the hardness of the products of the present invention is below the measurement limits of the Type A Shore Durometer, the hardness of the products described in U.S. Pat. No. 4,783,289 is readily measurable using such an apparatus. The tensile strength of the products exemplified in U.S. Pat. No. 4,783,289 ranges between 560-1420 psi and the elongation-to-break of the products range between 60-700%. The elongation of an article described in U.S. Pat. No. 4,783,289 increases with tensile strength. For example, an article with a tensile strength of 560 psi exhibits 60% elongation, whereas an article with a tensile strength of 1420 psi exhibits 600% elongation. By contrast, the products of the present invention exhibit tensile strength less than 20 psi, preferably less than 10 psi and most preferably less than 5psi and and elongation-to-break greater than 300% and often substantially greater than 500%, e.g. greater than 1000% even at tensile strengths less than 5 psi. In general, the products of the present invention are mechanically less robust than those described in U.S. Pat. No. 4,783,289. Forming substantially continuous shaped articles from such delicate materials of the present invention presents unique problems.
Thus, it would be highly desirable to have a method, and the articles fabricated therefrom, which can create simple, or complex, shapes at a rate and on a continuous basis for low cost commercial operations. In addition, it would be desirable to use a heated fluid, e.g. a silicone oil bath, to permit optimization of the surface properties of the gel product. Addition of additives to the heat transfer medium would also permit one to tailor the surface properties of the final article, for example by an optional final skinning operation, and the like.